In recent years, many portions of a vehicle are operated by electric motors, and thereby numbers of the electric motors and electronic control units thereof are increased. Many efforts for increasing effective inside space of the vehicle have been made in order to provide a comfortable space to a user. Accordingly, it becomes a problem to keep space for the electric motors and the electronic control units thereof. It is, therefore, an important issue for making the electric motors and the electronic control units thereof smaller in size.
For example, an electronic control unit used for an electric power steering system for the vehicle is arranged in a space of an engine compartment or behind an instrument panel. Since the electronic control unit for the power steering system drives the electric motor with large electric current (around 100 A), heat generated at switching elements is increased. Therefore, a high heat radiating structure is necessary in order to make the electronic control unit of this kind smaller in size.
In prior arts, for example, as disclosed in the following Japanese patent publications, an electric connecting pattern for heat radiation is provided on a printed circuit board so as to radiate heat of electric and/or electronic parts mounted on the printed circuit board. According to the prior arts, although the electric connecting pattern for the heat radiation is provided or electric and/or electronic parts are effectively arranged on the printed circuit board, desired heat radiating performance can not be obtained.